Preparing stock for coating and electroplating



Patented Sept. 28, 1943 PREPARING s'rocx FOR COATING AND nnnoraormrmoJohn S. Nachtman, Beaver, Pa.

' No Drawing. Application January 25, 1939,

Serial No. 252,78}

This invention relates to the art of coating metallic sheet, strip, wireor similar material, and is concerned with the steps or step ofpreparing the stock for reception of the coating. The process of theinvention will, hereinafter, be described with particular reference topreparation of ferrous metal strip stock for the recep- Claims. (Cl.204-34) prove the adherence of the electro-deposit with respect tov thebase stock.

Another object of the invention is the provision of a mode ofpreparation of the stock that will serve to improve the bond between thestrip and a metal coating, pl and/or electro-deposit tion ofelectrodeposit's; however, this procedural concept is not restricted toreception of electroplated coating but is applicable to the preparationof surfaces of metallic stock for reception thereon of metallicelectrodeposits generally, or for the reception of metallic coatings bya hot dip method, or for the reception of paints, enamels, lacquers,etc. i I In the plating art, particularly, in the art of tin plating, ithas been found desirable to skin roll the base metal prior to plating.If it is assumed that preliminary steps could be carried out perfectlyin such a manner that the strip is perfectly clean before skinrolling,and assumed that the rolls are perfectly smooth and hard, it would seemthat the electroplating could take place without further treatment ofthe strip. It has, however. been found that where skin rolling isconducted on a commercial scale, there is some lack of uniformity in thephysical or chemical condition of the surface of the strip after theskin rolling process, perhaps in part at least, due to rolling into thesurface small particles of foreign material which failed to wash off inthe preparation of thestrip. To obtain-a perfect bond between the stripand the coating material, the surface condition should be altered beforethe coating is applied.

Many treatments have been used in preparing metallic stock for thereception of coatings but none have fully attained the desired objectsof (1) insuring uniform adherence of the coating to the base stock; (2)bettering the surface smoothness of the final article. This has beenparticularly true where (as is usual), the strip or stock after beingcoated, is subjected to manufacturing operations involving manipulationof the same. Coatings applied to strip subjected to ordinary methods ofpreparation are found to peel and crack or otherwise separate from thebase metal, particularly, where the coated metal is subjectedtoprocesses such as forming, deep drawing, etc. i

It is an object of the present invention to provide a treatment of themetal stock or strip prior to coating that will serve to simultaneouslyprepare the stock for electroplating and'to imwhereby the latter willnot loosen on the former during subsequent alloying, brightening, and/orother manipulative treatments.

These and many other objects will appear to those skilled in the artfrom the description an the appended claims.

The above, and other objects of the invention are realized by procedurethat. involves the steps of: (1) subjecting the stock to a treatmenthaving as its result the formation of a very thin film-like and adherentcoating on the surface of the strip or stock; this coating can be anoxygen compound ofthe metal, a phosphate, a sulphide, or other compound,or a salt thatis adherent to the surface metal; and (2) subjecting thestock or strip to a treatment for suitably removin this thin skin orfilm-like coating.

I have discovered that by the carrying out of such a procedure the stockis admirably prepared for reception of subsequent electro-deposits.

The stock surface is altered in such a way that wise canbe effected withor without electrolytic action.

In accordance with my invention, I preferably positively andcontrollably induce a suitable uniform, restricted corrosion on themetal surface and then remove the corrosion product. The precise methodof inducing corrosion can be chemical, electro-chemical or thermal, andlikewise the precise method of removing the corrosion can be chemical,electro-chemical or thermal, as is illustrated by the following outlinesof sequence in carrying out the invention.

In accordance with one method, the strip is submerged in a suitablechemical solution until an adherent film-like coating is formed on itssurface, it is rinsed and subjected to a suitable chemical solutionuntil the film-like coating is removed, then it is wet mechanicallycleaned by subjecting thesurface simultaneously to rotating brushes andhot water sprays. This last action completely removes any residue leftby the chemical treatment, and leaves the surface suitable for receptionof electroplating. The liquid insuiates and protects the strip fromatmospheric gases, etc., prior to the coating application. In someinstances (where paint, lacquer, or enamel, etc., is applied to thesurface in place of electroplating) it is desirable to dry the stripbefore the final coating is applied.

A. As an example of the formation of an adherent film on the surface ofthe strip, I form an oxide on a ferrous metal stock by subjecting it toa bath of sodium hydroxide 40 to 50% by weight in water and maintainedat a temperature of 220 F., the ferrous metal stock being made anode ofa direct current electrolysis procedure employing any suitable (50 to300 amperes per square foot of surface) current density. The cathode maybe of any suitable electrically conducting material such as nickel oriron, which is not attacked by the solution. The treatment is preferablyterminated when there has been developed on the strip surface a chemicaloxide of iron coating of film thickness.

B. As an example of a solution for removing the film of chemical oxideof iron, I subject the coated stock to a bath of ferric chloridedissolved in water 100 grams per liter, and hydrochloric acid 2% byvolume at room temperature. Direct current electrolysis is practicedwith the strip as anode and a current density of approximately 10amperes per square foot of surface being suitable. Any suitableconducting material which will withstand the action of the solution maybe used as cathode. By way of example, I have used carbon. This solutionmay also be used without the aid of electric current by taking aslightly longer time to remove the oxide coating.

C. Another example of a bath for formation of the oxide is sodiumhydroxide 40% by weight in water, sodium nitrate 10 grams per liter,potassium nitrate 10 grams per liter.

D. Other examples of baths for removing the oxide coating arehydrochloric acid 5% by volume in water, or sulphuric acid 5% by volumein water.

E. As an example of the formation of oxide coating on ferrous metalstock by thermal means, I heat the ferrous stock in any suitable furnacein the presence of air and moisture. This may be at any suitabletemperature, say, IOU-800 F. The period of heating will be determined bythe gage of the material being processed and by the depth of oxide ofiron coating desired. The strip is then cooled. I have also provided adesired type and/or depth of oxide by controlling the moisture in theheating chamber; the material has also been air-cooled to furtherdesired results.

F. As an example of solution for removing the thermally applied oxidecoating, I subject the coated stock or strip to a solution of ferricchloride, 200 grams per liter in water plus hydrochloric acid 5% byvolume at room temperature, employing direct current electrolysis, thecoated stock being made anode with a current density of approximately 10amperes per square foot of when the strip emerges from the surface ofthe bath, while in the case of the fused salt, the oxide coating can beproduced directly in the bath in many cases, provided the molten saltcontains oxygen.

The oxide formed by the method outlined under G can be removed by any ofthe solutions as outlined under paragraphs B, D or F.

H. As stated above, an oxide coating can be applied chemically (e. g.,without current); as an example of this, I subject the strip to asolution consisting of the following:

Grams Water 600 Sodium hydroxide 400 Potassium nitrate 10 Sodium nitrate10 The solution temperature should be to centigrade.

consisting of the following:

Grams Water 200 Ferric chloride 10 Potassium ulphide 1 The stock andsolution thereon is dried and thereafter after rinsing, the film-likecoating can be removed by subjecting the coating to the treatment asoutlined under paragraph D. The acid pickle as specified under paragraphD can be used with or without electric current.

J. Similarly, a surface on metallic stock can be prepared for thereception of coatings by developing thereon a very thin phosphate filmfollowed by the removal of this phosphate film. For carrying out thisprocess, I treat the metallic stock in a phosphate bath containing freephosphoric acid or a soluble phosphate, and then remove the phosphatefilm coating by treating with an aqueous solution of ferric chloride 100grams per liter containing about 2% by volume hydrochloric acid.

K. A preparation treatment of a different sort than those described canbe effected as follows:

Steel strip is immersed in an aqueous solution of sodium thiosulphateand lead acetate of the following proportions:

The solution is used at a temperature up to 212 F. as a chemical dip,and the resulting lead oxide coating is removed by immersing theso-coated steel strip in an aqueous solution of citric acid, 100 gramsper liter, and employing direct current electrolysis using 10 amperesper square foot of surface being treated.

Such steps for preparing metallic stock for electroplating may beapplied to other metals than ferrous and may be employed regardless ofthe constitution of the coating thereafter applied to the material. Ofcourse, mechanical cleaning may be added to or combined with chemicalremoval of the metal oxide, salt, or compound, and other changes may bemade in the process without departing from the' spirit and scope of myinvention.

' The advantages obtained in the preparation of the base stock bymethods covered in this invention is that the adherence between the basestock and the coating is improved to such an extent that heaviercoatings may be applied. In the methods of preparation heretofore used,the strip was never considered for the reception of heavier coatingsbecause heavy coatings will crack and peel off during deep drawing andforming operations. In the preparation of metal stock as outlined inthis patent application, heavier coatings may be applied without thedanger of cracking and peeling off during deep drawing and formingoperations.

From the above description, it will appear that l the present inventionfirst deals with forming a suitable oxide or compound, and preferablythe latter, on surface portions of a metal shape such as a strip, sheet,plate, etc., that is to be aftercoated or plied with a metal. The metalcompound, oxide, etc., is preferably formed from adjacent surfaceportions of the strip being treated and the depth of the compound issuitably controlled in such a manner that a minimum thickness of thecompound is provided while at the same time a sufficient thickness isprovided to insure the removal of undesired materials, or in otherwords, to insure the desired treatment results. In the second place-themetal compound is removed in such a manner that undesired materials andparticles are, in effect, "skinned oif the surface portions to be coatedwithout destroying desired characteristics of adjacent surface portionsof the metal and while at the same time leaving exposed a surface thatis not only clean and free from imbedded particles, but that is also inwhat may be termed a coating receptive or nascent condition.

The present invention is the result of certain discoveries that I havemade. I determined that metal articles, strip, etc., not only have acertain amount of oxide, oil, and other materials on immediate surfaceportions thereof, but also have small extraneous particles of undesiredmaterials worked into the grain structure adjacent surface portionsthereof; that is, the preworking and pretreatment processing of thematerial tend to introduce extraneous and undesired materials to acertain depth within surface portions of the material which additionallymakes the material unsuitable for subsequent coating applications.

In developing the present invention, I also experimented with the ideaof removing an oxide such as deliberately produced by a thermal methodwith an annealing or reducing treatment such that the original metal isretained. However, I determined that such a procedure did not eliminatethe extraneous particles imbedded adjacent surface portions of thearticle, and further, that the crystalline structure or molecular natureof the treated portions of the metal surface has undesirablecharacteristics imparted to it. The surface thus produced, althoughcleaned of greases, etc., was somewhat porous-like and/or roughened innature and did not take a coating as satisfactorily as desired. In otherWords. the preworking or conditioning of the crystalline structure ofthe strip was spoiled at least adjacent its degreased surface portions.

I discovered that a highly suitable type of surface for subsequentcoating could be obtained without altering the mechanical or crystallinenature of the surface portions of the sheet to be coated by firstforming a metal skin and then peeling oil or removing that skin in sucha manner as to remove the oxides, oils, and other extraneous particles,and to expose a bright'coating-sensitive or receptive under surfacehaving all the desired mechanical and/or crystalline characteristicsthat were previously imparted thereto by pretreatment or preworking,etc. Further, in accordance with the present invention, I prefer toimmediately coat the thus-exposed under surface portions of the materialbefore they become exposed to the atmosphere; in this connection,depending upon the type of coating treatment, the sheet after theremoval of its skin can be introduced into a treating bath through aconduit containing a non-oxidizing or reducing atmosphere or may becovered with a suitable insulating material such as water. The formedsurface skin'is removed, in accordance with myinvention, by chemical, orelectro-chemical, and mechanical means. The mechanical means may consistof suitably driven rotating brushes combined with fluid spray means(preferably water or steam) for simultaneously washing and brushing. Thelatter method is suitable particularly where the strip is to beelectroplated and has been successfully employed to complete the removalof a surface skin that' has been preliminarily or substantially removedby chemical, electro-chemical, or other suitable treatment. The wateremployed will protect the I material from film forming gases such asulphur dioxide prior to the immersion of the piece in an electroplatingbath.

As previously intimated, the present invention is suitable forconditioning a piece of material such as a metal strip in order toobtain a much improved type of metal coating application thereto. Thecoating can be applied by hot dipping, electroplating, or other suitablemeans. Further, this type of conditioningjs also advantageous when thestrip or material is to be enameled, painted, etc., with or without ametal coating application thereto. The present invention produces abright, smooth, and uniform surface; thus, it is apparent thatafter-treatment cf the surface such as by painting or coating need notnecessarily follow, as the invention in general applies to a surfaceconditioning of metal strips, etc. I

invention, I-have referred to certain specific examples, it will beapparent to those skilled in the 'art that many modifications,alterations,

the coating from the ferrous metal base, fromcommercially cold rolledferrous metal material having foreign material on and rolled into thesurface thereof and having physical and chemical surface conditions ofsuchlack of uniformity that a uniformly adherent, perfectly bonded,smooth coating of tin whichv will not peel, crack or separate therefromwhen formed or deep drawn, normally cannot be applied thereto; thest'epsin the order named of forming onthe surface of said Although for thepurpose of illustrating my I commercially cold rolled ferrous metalmaterial a uniform, thin, film-like, adherent coating of a compound ofsaid ferrous metal from said ferrous metal material of the classconsisting of iron oxide, iron sulphide, and iron phosphate; thensubjecting the film-like coated ferrous metal material to the action ofan aqueous solution of an acid of the group consisting of hydrochloricand sulphuric acid to dissolve said film-like coating; then wetmechanically cleaning the same to remove said fihn-like coatingcompletely therefrom; the film-like coating serving as a vehicle, thedissolution and removal of which is accompanied by removal of foreignmaterial on and rolled into the surface of said commercially cold rolledferrous metal stocktthereby exposing a clean, smooth, bright, uniformferrous metal surface, free from overlying or imbedded foreign material,and of nascent or coating receptive condition, and having the mechanicaland crystalline characteristics previously imparted thereto by coldrolling; and then immediately applying to said coating receptive surfaceof said ferrous metal material a coating of tin before exposing thecoating receptive surface to the atmosphere.

2. In a method of making coated ferrous metal material having a ferrousmetal base and a uniformly adhering, perfectly bonded, smooth coating oftin, adapted for being formed or deep drawn without peeling, cracking orseparation of the coating from the ferrous metal base, from commerciallycold rolled ferrous metal material having foreign material on and rolledinto the surface thereof and having physical and chemical surfaceconditions of such lack of uniformity that a uniformly adherent,perfectly bonded, smooth coating of tin which will not peel, crack orseparate therefrom when formed or deep drawn, normally cannot be appliedthereto; the steps in the order named of forming on the surface of saidcommercially cold rolled ferrous metal material a uniform, thin,film-like, adherent coating of a compound of said ferrous metal fromsaid ferrous metal material of the class consisting of iron oxide, ironsulphide, and iron phosphate; then subjecting the film-like coatedferrous metal material to the action of an aqueous hydrochloric acidsolution including ferric chloride to dissolve said film-like coating;then wet mechanically cleaning the same to remove said film-like coatingcompletely therefrom; the film-like coating serving as a vehicle, thedissolution and removal of which is accompanied by removal of foreignmaterial on and rolled into the surface of said commercially cold rolledferrous metal stock, thereby exposing a clean, smooth, bright, uniformferrous metal surface, free from overlying or imbedded foreign material,and of nascent or coating receptive condition, and having the mechanicaland crystalline characteristics previously imparted thereto by coldrolling; and then immediately applying to said coating receptive surfaceof said ferrous metal material a coating of tin before exposing thecoating receptive surface to the atmosphere.

3. In a method of making coated ferrous metal material having a ferrousmetal base and a uniformly adhering, perfectly bonded, smooth coating oftin, adapted for being formed or deep drawn without peeling, cracking orseparation of the coating from the ferrous metal base, from commerciallycold rolled ferrous metal material having foreign material on and rolledinto the surface thereof and having physical and chemical surfaceconditions of such lack of uniformity that a uniformly adherent,perfectly bonded, smooth coating of tin which will not peel, crack orseparate therefrom when formed or deep drawn, normally cannot be appliedthereto; the steps in the order named of forming on the surface of saidcommercially cold rolled ferrous metal material a uniform, thin,film-like, adherent coating of iron oxide from said ferrous metalmaterial by subjecting the ferrous metal material to the action of a hotaqueous solution of sodium hydroxide; then subjecting the film-like,

iron oxide coated ferrous metal material to the action of any aqueoussolution of an acid of the group consisting of hydrochloric andsulphuric acid to dissolve said film-like coating; then wet mechanicallycleaning the same to remove said film-like coating completely therefrom;the filmlike coating serving as a vehicle, the dissolution and removalof which is accompanied by removal of foreign material on and rolledinto the surface of said commercially cold rolled ferrous metal stock,thereby exposing a clean, smooth, bright, uniform ferrous metal surface,free from overlying or imbedded foreign material, and of nascent orcoating receptive condition, and having the mechanical and crystallinecharacteristics previously imparted thereto by cold rolling; and thenimmediately applying to said coating receptive surface of said ferrousmetal material a coating of tin before exposing the coating receptivesurface to the atmosphere.

4. In a method of making coated ferrous metal material having a ferrousmetal base and a uniformly adhering, perfectly bonded, smooth coating oftin, adapted for being formed or deep drawn without peeling, cracking orseparation of the coating from the ferrous metal base, from commerciallycold rolled ferrous metal material having foreign material on and rolledinto the surface thereof and having physical'and chemical surfaceconditions of such lack of uniformity that a uniformly adherent,perfectly bonded, smooth coating of tin which will not peel, crack orseparate therefrom when formed or deep drawn, normally cannot be appliedthereto; the steps in the order named of forming on the surface of saidcommercially cold rolled ferrous metal material a uniform, thin,film-like, adherent coating of iron sulphide from said ferrous metalmaterial by subjecting the ferrous metal material to the action of asolution consisting of ferric chloride, potassium sulphide and water;then subjecting the film-like iron sulphide coated ferrous metalmaterial to the action of an aqueous solution of an acid of the groupconsisting of hydrochloric and sulphuric acid to dissolve said film-likecoating; then wet mechanically cleaning the same to remove saidfilm-like coating completely therefrom; the film-like coating serving asa vehicle, the dissolution and removal of which is accompanied byremoval of foreign material on and rolled into the surface of saidcommercially cold rolled ferrous metal stock, thereby exposing a clean,smooth, bright, uniform ferrous metal surface, free from overlying orimbedded foreign material, and of nascent or coating receptivecondition, and having the mechanical and crystalline characteristicspreviously imparted thereto by cold rolling; and then immediatelyapplying to said coating receptive surface of said ferrous metalmaterial a coating of tin before exposing the coating receptive surfaceto the atmosphere.

5. In a method of making coated ferrous metal material having a ferrousmetal base smooth coating of tin,

formed or deep drawn without peeling, crack- V and a uniformlyadhering," perfectly bonded,

adapted for being ing or separation of the coating from the ferrousmetal base, from commercially cold rolled ferrous metal material havingforeign material on and rolled into the surface thereof and havingphysical and chemical surphosphoric acid; then subjecting the film-likeiron phosphate coated ferrous metal material to the action of an aqueoushydrochloric acid solution to dissolve said film-like coating; then wetmechanically cleaning the same to remove said film-like coatingcompletely therefrom; the filme like coating serving as a vehicle, thedissolution and removal of which is accompanied by removal of foreignmaterial on and. rolled into the surface of said commercially coldrolled ferrous metal stock, thereby exposing a clean, smooth, bright,uniform ferrous metal surface, free from overlying or imbedded foreignmaterial, and of nascent or coating receptive condition, and having themechanical and crystalline characteristics previously imparted theretoby cold rolling; and then immediately applying to said coating receptivesurface of said ferrous metal material a coating of tin before exposingthe coating receptive surface to the atmosphere, 1

JOHN S. NACHTMAN.

